Rather than spectral reflectance, Ivushkin et al.used satellite thermography to assess soil salinity in salt-affected cropped areas in a semi-arid province of Uzbekistan.They found that correlations between soil salinity and canopy temperature varied depending on the time of year with the strongest relation occurring for cotton in September.The thermographic approach has also been applied to larger regional-and global-scales.Remote sensing of salinity has moved beyond proof-of-concept, but few salinity monitoring programs utilize satellite RS.One exception is the Land Monitor under the National Dryland Salinity Programin Australia, which tracks salinity in Western Australia.However, further research is needed to establish that RS is sufficiently accurate and cost effective for more general use.We recognize several priorities: Data integration—With satellite imagery, trade-offs exist among spatial, temporal, spectral, and radiometric resolutions.Satellites and instruments used for indirect remote salinity detection include Landsat 7 ETM+and Aqua/Terra MODIS.The most recent iterations of long-operating open satellite platforms offer improved imaging capabilities while commercial satellites such as WorldView-3 offer spatial resolutions approaching 1m.Research is needed to integrate data from these varied platforms and technologies because each potentially captures information important for salinity detection.High temporal resolution is important because spectral and thermal response varies according to phenological stage.High spatial resolution is important because salinity often varies substantially over very short distances.However, the finest possible resolution is not necessarily optimal, as correlation between remotely sensed data and soil properties may be highest at coarser resolutions.For instance, Scudiero et al.used data from the WorldView-2 satellite to examine salinity correlations in a 34ha fallow field and determined that the relationship between multi-temporal maximum EVI and soil salinity was strongest at a resolution of about 20m.
Future research should develop multi-spatial,aeroponic tower garden system multi-temporal, multi-sensor data analysis pipelines to improve accuracy.Crop-specific information—Research should prioritize regression and classifier models that integrate crop-specific data.As noted, spectral and thermal response to salinity stress differs by vegetation type and growth stage, but very few RS salinity models have used crop specific crop data.Exceptions include the work by Scudiero et al.who used the Cropland Data Layer to incorporate cropping statusin to their model, and Zhang et al.who explored the possibility of incorporating crop-specific reflectance properties in their regional salinity assessments.Future research should investigate the use of crop type and growth stage as predictor variables.Two crop categories that create difficulties for indirect remote sensing methods are salt-tolerant halophytes and orchards and vineyards.Halophytic vegetation complicates image analysis because in contrast to the monotonically decreasing salinity response function of most agronomic crops, halophytes achieve maximal growth at intermediate salinity levels.While most true halophytes have little agronomic value, there is growing interest in their use as bio-fuels.Orchards and vineyards are mostly excluded in salinity RS studies.For example, the salinity map of western San JoaquinValley produced by Scudiero et al.covered only row and field crops because insufficient information existed for orchards.Hyperspectral imagery—Multi-band vegetation indices have been the predominant measure of canopy reflectance in RS studies.However, hyperspectral imagery potentially provides a more informative measure of crop status as potentially 100s of wavelength bands can be analyzed simultaneously.Irrigating with water that is high in salt content requires special management practices to mitigate salinity buildup in the crop rooting zone, to minimize reduction in crop yield with associated economic losses and to mitigate environmental degradation.In addition, saline-sodic irrigation water can cause breakdown of soil aggregates, followed by the swelling and dispersion of clays particles which leads to soil crusting, loss of porosity and reduced permeability especially after rainfall or irrigation with low salinity.The degradation of alkali soils using high quality irrigation waters has been documented early on, resulting in reduction in soil infiltration.We will discuss the historical evolution of improved soil salinity management practices in irrigation projects, followed by changes in soil salinity management strategies that have occurred in the past few decades.
Early on with the development of irrigation projects, there was general recognition that soil salinity issues had to be addressed at both the on-farm scale and at the basin or irrigation district scale.On the farm scale the focus was on agronomic and engineering practices that minimized soil salinity buildup in the root zone, while at the basin or regional scale the focus was mostly on engineering structures for water delivery and drainage.In our review we will focus on the farm-scale only, though it is realized that with few exceptions soil salinity issues will persist when regional efforts to ensure adequate drainage facilities are lacking, and will eventually lead to the demise of civilizations and land abandonment.We also note that most irrigation projects were designed for surface irrigation by flooding the field using gravity , allowing for over-irrigation to ensure that the whole field receives adequate amounts of water while satisfying the annual leaching requirement.However, this has led to rising groundwater tables worldwide, further necessitating drainage capabilities.At the same time, these shallow groundwater tables can be beneficial when irrigation water supplies are limited such as in drought periods.A succinct review by Ayers et al.lists main criteria to assess whether in situ crop water use from shallow ground water is suitable.To prevent the buildup of salts in the root zone, agronomic recommendations would apply irrigation water in excess of crop evapotranspiration.The excess water was commonly referred to as the leaching requirement, maintaining a field salt balance with soil salinity levels to not exceed the crop salt tolerance.In situations when leaching was inadequate to prevent salt buildup in the root zone, salt tolerant crops were selected.Seedbed preparation by tillage and higher frequency irrigation were used for sodic soils to mitigate the effects of surface crusting and to promote stand establishment.However, tillage can reduce soil infiltration through formation of a plow layer.For that purpose, deep plowing is used to break the plow pan and to increase leaching and soil water storage in the deep rooting zone.Other soil salinity management strategies included sanding, by mixing clay layers with sand from further down below, thereby improving the effectiveness of leaching, or by creating artificial subsurface barriers.
Flood irrigation, though suited for irrigation with saline water because of its leaching benefit, is often associated with problems such as soil crusting and soil aeration.These are minimized using furrow irrigation, however, because of its partial wetting of the soil surface it tends to accumulate salts in the seedbed.For that purpose, annual preplant irrigations by flooding or sprinklers were applied to flush salts from the shallow root zone before or during seedling establishment.Chemical amendments are used to replace the excess exchangeable sodium with calciumin sodic soils to improve soil infiltration.In addition to gypsum, other amendments include calcium chloride, sulfur, and lime.Addition of such amendments is typically followed with a leaching irrigation to move Na and other reaction products downwards away from the rooting zone.Soil conditioners continue to be used for management of saline-sodic soils,dutch buckets for sale particularly at seedling establishment in high ESP soils or when crops are irrigated with high SAR water.Soil conditioners such as sulfate lignin were reported to improve soil aggregate stability and permeability and prevent crust formation.Also, organic manures are used to manage saline-sodic soils irrigated with lower quality water, as these promote soil aggregation and increase soil permeability.Organic manures arealso used to lower soil pH by releasing CO2 and organic acids as it decomposes, whereas the lower pH helps in solubilizing CaCO3 when present, thereby increasing soil EC and replacing the exchangeable Na with Ca which lowers ESP.In the last few decades, substantial changes have occurred in irrigation technology, irrigation water sources and cropping systems.Also, public awareness on environmental issues and their regulations have increased.Consequently, soil salinity management is changing as well.Leaching—Leaching remains an effective management strategy to prevent salt build in the root zone.However, more recent research is showing that soil salinity leaching requirements developed decades ago were based on steady state conditions and that the transient models developed later improved the prediction of the complex physiochemical-biological dynamics in an agricultural system.They concluded that the current guidelines overestimated leaching requirements , especially if LR are low.Most importantly, the salt concentration at a given depth is not constant with time as assumed by steady-state models, but is continually changing as water is added or extracted by the plant.Furthermore, under monsoonal conditions, rainwater mobilizes accumulated salts downwards and restores high quality soil water in the rooting zone during the growing season, thus further reducing the LR as computed by the steady state model.The concentrated salts near the soil surface are “flushed” by the irrigation water thereby moving the salts downwards and reducing the concentration at a given depth.
As a result, the concentration after irrigation near the soil surface would be close to the concentration of the irrigation water for high-frequency irrigation systems.Such findings indicated irrigation water amounts could be reduced and that more saline waters and marginal waters could potentially be used for irrigation.These results were affirmed by Corwin et al.and Corwin and Grattan.In addition, using both field experiments and transient numerical modeling studies, Hanson et al.showed that there is considerable localized leaching around drip systems, even at applied water volumes less than potential crop ET, as drip systems only partially wet the soil surface.Deficit irrigation —DI consists of application of irrigation water below potential crop requirements.DI strategies such as partial root zone drying and regulated DI are used to save water and increase water productivity but will increase soil salinity when annual LF values are less than one.In a 5-year field study on peach trees, Aragu€es et al.determined that this increase was counteracted by salt leaching by high LFs attained during the non-irrigation seasons and proved to be sustainable for the climatic conditions of their study area.However, in a similar study using low-quality irrigation water they determined that long-term application of moderate saline waters would increase soil salinity in the long-term, unless unusual large volumes of irrigation water were applied in the non-irrigation season.Clearly, long-term outcomes of DI will largely depend on crop salt tolerance and climatic conditions.Crop selection—Selecting salt tolerance crops continues to be used as a simple strategy to deal with saline-sodic soils irrigated with low quality water.For example, in the western San Joaquin valley cotton production has been replaced by pistachio, which is both salt tolerant and a high value specialty crop.However, in general there are not that many crop choices that are both salt tolerant and high value as most fruit and vegetables tend to be salt sensitive, such as lettuce and strawberries.Boron and chloride ion toxicity on woody perennials is occurring more frequently as acreage of this crops is expanding in California.Typically, more water is needed to leach boron than other salts because it is tightly adsorbed on soil particles , whereas tolerances vary among species and root stocks.Boron concentrations in the irrigation water exceeding 0.5–0.75mg/L have been reported to reduce plant growth and yield.Unlike boron, chloride moves readily with the soil water, is taken up by the plant roots, translocates to the shoot, and accumulates in the leaves.If irrigation water that is high in chloride is applied via sprinkler irrigation it can cause foliar injury and reduce yields in hot climates.Options to reduce foliage injury include irrigation at night or early morning when evaporation rates are low and using infrequent and large irrigation applications.Effect of irrigation systems on soil salinity management—The soil salinity pattern that develops in the root zone is a function of the water distribution pattern of a given irrigation system.Over the last 2 decades, there has been a rapid conversion from surface irrigation to pressurized irrigation systems particularly drip irrigation in places like California.The rapid increase in adoption of drip irrigation has been driven by both the demonstrated ability to improve productivity and water use efficiency, as well as it is incentivized by governments.Surface irrigation systems remain the most widely used method of irrigation around the world.Recent advances in automation and real-time data analytics for surface irrigation have demonstrated improved water use efficiency in Australia and California.Distributing applied water more uniformly across the field results in leaching of salts with less water.But traditionally, surface irrigation systems such as flood have typically had lower leaching efficiencies than micro-irrigation systems, because under soil saturation large fractions of applied water move through macropores thereby bypassing the salts in the smaller pore spaces of the soil matrix and aggregates.However, automated gates and SCADA control systems can now allow flood irrigation systems to achieve leaching efficiencies like pressurized irrigation systems.